exhaust back pressure

[Ratt_Bones]

RBones

Explain how opening the muffler up so the engine can breathe betrer is a bad idea?

some of us here have several years experience not only with these engines but many others, and ill tell you one thing you left out or just dont know.

Carb tuning.......

If the carb is rejetted for the extra air flow through the engine a wide open exhaust wont hurt it in any way, as with any engine, extra air flow requires the fuel flow to be increase at the same time so that you dont have a lean run condition which causes the engine to run much higher temps which could potentially fry the engine, but if the engine gets a fuel flow increase that coincides with the added air flow what you get is a better running engine and more power.

The only life shortening effect that will be realized by doing this is that the rpm capability of the engone will increase and that and only that will have potential to shorten its life because now you can run it a little harder.

Not intending to be abrasive with this reply to what you said in your post but a good way to get some abrasive comment pointed your way on here is to speak as though one is an authority on something when the experienced know what you said is not the fact of the matter.

Ever been involve in Drag racing? They run open headers..... and the air/fuel mix is adjusted accordingly. ... I personally dont know of any engine that works any different.

Peace, map

It improves cylinder filling of the air/fuel mixture...if you lower back pressure the air/fuel charge does not fill the combustion chamber as well, but simply flows through, resulting in less power. It's like filling a bucket with water....if the bucket has a huge hole in it, it takes longer to fill it. Back pressure is like a wall in the exhaust port that the air/fuel mixture hits when entering the combustion chamber so it fills up more....resulting in more complete filling and more power on detonation. There are other important considerations regarding back pressure as well, but this is more important in these smaller engines

In drag racing, those engines are designed to go very fast in one direction for less than ten seconds, but yes I am familiar. I had owned and operated a hot rod shop for a long time before retiring from it.

It's ok, I didn't take it abrasive. I am sort of new to motorized bicycles, but not to engine tuning. I do know this though as far as my experience goes with these mufflers. I had a stock/unmodified muffler, my bike running at 43mph as it sat. I hit some railroad tracks and the muffler broke after the header. A buddy of mine lent me his muffler that he had been bragging about (same one) that he had done the same above mods to. After swapping them out, I can't get above 30mph. Although my bike sounds way better, it just lacks the proper back pressure. I'm not going to bother retuning the carb, but just wait and get a new pipe.

No worries about coming off as abrasive. I appreciate intelligent replies and feedback, rather than some one telling me that if it sounds better it must be better or something like that. LOL

 

[mapbike]

It improves cylinder filling of the air/fuel mixture...if you lower back pressure the air/fuel charge does not fill the combustion chamber as well, but simply flows through, resulting in less power. It's like filling a bucket with water....if the bucket has a huge hole in it, it takes longer to fill it. Back pressure is like a wall in the exhaust port that the air/fuel mixture hits when entering the combustion chamber so it fills up more....resulting in more complete filling and more power on detonation. There are other important considerations regarding back pressure as well, but this is more important in these smaller engines

In drag racing, those engines are designed to go very fast in one direction for less than ten seconds, but yes I am familiar. I had owned and operated a hot rod shop for a long time before retiring from it.

It's ok, I didn't take it abrasive. I am sort of new to motorized bicycles, but not to engine tuning. I do know this though as far as my experience goes with these mufflers. I had a stock/unmodified muffler, my bike running at 43mph as it sat. I hit some railroad tracks and the muffler broke after the header. A buddy of mine lent me his muffler that he had been bragging about (same one) that he had done the same above mods to. After swapping them out, I can't get above 30mph. Although my bike sounds way better, it just lacks the proper back pressure. I'm not going to bother retuning the carb, but just wait and get a new pipe.

No worries about coming off as abrasive. I appreciate intelligent replies and feedback, rather than some one telling me that if it sounds better it must be better or something like that. LOL

R bones, you have made good points here and if you have a stock engine that will run 43mph with a stock pipe, all I can say is you have one heck of a unique setup in my experience, you better hope that hing runs for years because I have nev r personally had a kit out of the box that would do that mph with the choked down stock pipe that doesn't even come close to being port matched to the exhaust port, some are better out of the box than others but you have a real fire cracker of a set up if you have good pulling power and 43mph speed Outta yours.

As you work with these engines you will learn that they dont act like a high end precision engine, some principals apply universally of course, but I have never had one yet that perfromed even remotely as good with a stock pipe as they did with a less restrictive exhaust, my fastest bike right now has seen 45mph and I know for a fact it wouldnt do anything close to that with a stock pipe.

I put a stock pipe on one of my bikes a while back just to see if I could quiet it down and not loose much, my results were that a bike that was topping out at 38mph would only register 29mph on the flat and lost some much low end pulling power that it was sad, and it ran its best with a home built pipe that has very little back pressure, I am not arguing with what you have said you results are, but all I have to go by is my own results of running and tuning these engine over the last 4 years and the results of several other members here that ditched the stock pipes and seen speed increase and much beter hill pulling power, I run an expansion pipe for making good power but not restriction in the exhaust like the stock pipes create.

Best of luck with getting everything back to good on with your bike.

Map

 

[Ratt_Bones]

R bones, you have made good points here and if you have a stock engine that will run 43mph with a stock pipe, all I can say is you have one heck of a unique setup in my experience, you better hope that hing runs for years because I have nev r personally had a kit out of the box that would do that mph with the choked down stock pipe that doesn't even come close to being port matched to the exhaust port, some are better out of the box than others but you have a real fire cracker of a set up if you have good pulling power and 43mph speed Outta yours.

As you work with these engines you will learn that they dont act like a high end precision engine, some principals apply universally of course, but I have never had one yet that perfromed even remotely as good with a stock pipe as they did with a less restrictive exhaust, my fastest bike right now has seen 45mph and I know for a fact it wouldnt do anything close to that with a stock pipe.

I put a stock pipe on one of my bikes a while back just to see if I could quiet it down and not loose much, my results were that a bike that was topping out at 38mph would only register 29mph on the flat and lost some much low end pulling power that it was sad, and it ran its best with a home built pipe that has very little back pressure, I am not arguing with what you have said you results are, but all I have to go by is my own results of running and tuning these engine over the last 4 years and the results of several other members here that ditched the stock pipes and seen speed increase and much beter hill pulling power, I run an expansion pipe for making good power but not restriction in the exhaust like the stock pipes create.

Best of luck with getting everything back to good on with your bike.

Map

Nah, When I said "as it sits", that was with all the current mods. However, I still have a stock exhaust. I'm also running a different gear ratio than stock. My low end does suck, no denying that.... But I live in Wisconsin. Not much in the way of hills here so I had little in the way of concern for low end torque. It does take a little to get up to speed, but in my situation I'm good with that.

One of the other things that is important with back pressure on these engines is that it does help to keep them lubricated. It might not seem like much, but every bit helps.

I am going to replace my stock exhaust with a race expansion chamber from Honda, and use some flex pipe for header. Then use some Wrap and a silencer to quiet it down a bit. My goal is to break 50mph. It most likely will never happen, but you gotta have goals right?

 

[mapbike]

Nah, When I said "as it sits", that was with all the current mods. However, I still have a stock exhaust. I'm also running a different gear ratio than stock. My low end does suck, no denying that.... But I live in Wisconsin. Not much in the way of hills here so I had little in the way of concern for low end torque. It does take a little to get up to speed, but in my situation I'm good with that.

One of the other things that is important with back pressure on these engines is that it does help to keep them lubricated. It might not seem like much, but every bit helps.

I am going to replace my stock exhaust with a race expansion chamber from Honda, and use some flex pipe for header. Then use some Wrap and a silencer to quiet it down a bit. My goal is to break 50mph. It most likely will never happen, but you gotta have goals right?

Sounds like a plan on the exhaust, not to sure about the back pressure makes it lube better idea but, hey we all have our Ideas......lol!

50mph is not impossible because it has been done by bairdco for sure.

I have a 34T on my fastest bike and it still pulls my 215lbs up the hills around here where I live good, I've got a 30T im thinking about putting on it just to see what I will have on the flat....might be a dawg! against a head wind and on the hills, but when I get my carb upgrade done, my new expansion exhaust built and my new custom built high performance cdi and coil which by the way was built for me by a good friend here on the forum I might be able to get away with the 30T and might even knock on that 50mph mark but if not I may end up with a flat out cruise speed of 45-47mph if my engine will pull that gear.

Time will tell.....shrug?

Map

 

[dodge dude94]

I went TL;DR on you guys but I didn't see anything about the fact that most of the mufflers now are catalytic mufflers, meaning they have ZERO performance benefit. The only thing the back pressure does is keep the oil mixture from blowing right now the pipe. If you were talking about an x-pipe I'd agree with you, but if you're running a stock cat-muffler, as I am, and your plug is coming out good, you've got nothing to worry about dropping the back pressure a bit.

 

[Ratt_Bones]

I went TL;DR on you guys but I didn't see anything about the fact that most of the mufflers now are catalytic mufflers, meaning they have ZERO performance benefit. The only thing the back pressure does is keep the oil mixture from blowing right now the pipe. If you were talking about an x-pipe I'd agree with you, but if you're running a stock cat-muffler, as I am, and your plug is coming out good, you've got nothing to worry about dropping the back pressure a bit.

I have to agree with you on that. Those catalytic POS'S are exactly that.